Getting married combines their salaries so they can invest
Instead poorer parents focus on keeping their children safe and healthy.
And poorer children are more likely to be brought up by a single parent than their wealthier peers.
The US research looked at the changing role of marriage and how it affects the nurturing of kids.
It found mothers at all economic levels spend more time with their children now than was common 30 years ago.
But married richer parents are more likely to stay together for the sake of the children while poorer ones are more likely to cut their losses and separate.
Professor Shelly Lundberg, of the University of California, Santa Barbara, said: “In terms of time and money the well educated, higher income parents have increased their investments in children much more than those with lower incomes.
“They have the know how and the resources and they expect to help their children become economically successful in a way that may seem out of reach for parents with much lower levels of resources.”
Previous UK research found less educated women born in the 60s have a divorce rate 30 per cent higher than that of the better educated.
They are also more likely to be lone parents – with figures and trends even more pronounced in the US.
In the UK the baby-boomer generation has maintained a healthy level of marriage, with 87 per cent of men and 92 per cent of women having married at some stage.
But subsequent generations are facing a sharp decline in rates with half of 20-year-olds saying they will never marry.
In the 1960 over seven in ten Americans were married but by 2010 t just half were.
Added Prof Lundberg: “When the joint project of intense investments in children seems out of reach, it may not seem worth putting up with the disadvantages of marriage.
“One possible implication if we are right, and I should say that this is a speculative argument, is it may be possible to encourage investing in children among lower income parents by devoting more social resources to early childhood, enabling parents to see a brighter future for their children.
“These societal investments could, in turn, make longer term commitments among these parents more feasible and advantageous.”
Since the 1950s, the number of couple exchanging “I dos” has dropped steadily and while most Americans do marry at some point in their lives, many are putting it off till later.
The study published in the journal The Future of Children says marriage has morphed from an institution based on gender where the man was the breadwinner and the woman reared the kids to a means of supporting intensive investment
Prof Lundberg explained: “In a gender specialised economy
“But when women’s educational attainment increased and surpassed that of men, and women became more committed to jobs and careers, the kind of economic disparity that supported a division of labour in the household eroded.”
But while marriage rates have declined consistently over time, they have fallen far less among those with professional degrees than the less well educated.
Also graduates tend to marry before they start families and, when they do wed, their marriages are more stable than those of couples with less education.
But one aspect of marriage that hasn’t changed much over the years is that most men and women eventually do marry.